1. Field of the Invention
The present invention relates to a semiconductor doped with impurities, and in particular to a semiconductor doped with impurities with high concentration and a method for fabricating such a semiconductor.
2. Description of the Related Art
The high-concentration doping of impurities is an important technique for semiconductor devices. Compounds such as GaAs and AlGaAs used for fabricating high-speed devices, for example, are doped to the extent of 3 to 5.times.10.sup.18 /cm.sup.3 using Si as n-type impurities with a low diffusion coefficient and high in thermal stability. Such a high-concentration doped layer is used as an electrode lead-out layer (GaAs) for the field effect transistor or the bipolar transistor and also as an electron donor layer (AlGaAs) for the hetero-junction field effect transistor utilizing the modulation doped structure, and thus has contributed to an improved device performance.
With the recent trend to an ever-smaller size and an ever-higher performance of the devices, however, a reduced element resistance is assuming an increasing importance. Under the circumstances, there is a growing demand for reduction in the channel resistance of the hetero-junction field effect transistor and in the electrode contact resistance of various transistors. Thus, an even higher-concentration doping is sought after.
The impurities doping has a metallurgical solution limit. In the case where the solution limit of impurities is exceeded in a semiconductor material providing a base, the impurity induced phase separation or extreme deterioration of crystallinity is caused. The solution limit of various impurities has been examined with comparative frequency for representative semiconductors.
With an impurities concentration far lower than the metallurgical solution limit, on the other hand, it is actually known that the saturation of electron concentration occurs.
Although the Si impurities are dissolved in GaAs to the extent of 5.times.10.sup.19 /cm.sup.3 or more, for example, it is generally known that about 5.times.10.sup.18 /cm.sup.3 is an actual limit as an active donor. The Si concentration coincides with the electron density for lessthantheactual limit value. Fortheactual limit or more, however, the electron concentration is known to saturate or assume a lower value in spite of an increased Si concentration.
This concentration saturation has been studied for various semiconductors. Tokumitsu, et al. have studied the maximum electron concentration for the various semiconductors reported in the past and have discovered that the value of the maximum electron concentration is closely related to the charge neutrality level, which is referred to as the "Fermi level stabilization energy" in the Tokumitsu, et al. papers, and the energy position of the conduction band or the valence band in each semiconductor. (Japanese Journal of Applied Physics, Vol.29, No.5, L698-L701, 1990)
From the result of the above-mentioned study, it has been found that the saturation of the electron (hole) concentration in a semiconductor has a close relation with the Fermi level of the particular semiconductor system. Specifically, although the Fermi level of an n-type semiconductor increases due to the doping, actually there exists an upper limit of the Fermi level, beyond which any effort of doping results in the compensation effect due to the generation of an inherent defect and the electron concentration is saturated virtually at a fixed value.
This model explains clearly the well-known fact that the saturation concentration of a semiconductor having a large forbidden gap is small and that the compensation phenomenon due to an inherent defect (self-compensation effect) is liable to occur. In fact, experiments have also confirmed that high-concentration Ga vacancy or related defects (acceptors) are generated with the increase in the doping concentration of GaAs or AlGaAs.
Also, even in the case where Si is added to GaAs at the saturation concentration or more, for example, the vacancy or related defects are not generated and Si enters the donor site by adding acceptors and thus reducing the Fermi level at the same time. This substantiates the importance of the Fermi level for activation of Si as a donor.
The empirical formulae reported by Tokumitsu, et al. are described below.
In the case of n-type semiconductor: EQU n/N.sub.C =2.7.times.10.sup.3 exp(-5.5(E.sub.C -E.sub.FS)) (1)
In the case of p-type semiconductor: EQU p/N.sub.V =4.0.times.10.sup.3 exp(-6.1(E.sub.FS -E.sub.V)) (2)
where n is the effective donor concentration (in cm.sup.-3), p is the acceptor concentration (in cm.sup.-3), N.sub.c is the effective density of states of the conduction band (in cm.sup.-3), Nv is the effective density of states of the valence band (in cm.sup.-3), E.sub.C is the energy value (in eV) from the upper end of the valence band to the lower end of the conduction band, and E.sub.FS is the energy value (in eV) from the upper end of the valence band to the charge neutrality level.
Also, E.sub.V indicates the energy value at the upper end of the valence band. Since this energy value is used as a reference, it is given as E.sub.V =0 (eV). The value of E.sub.FS -E.sub.V is given from Table 1 (p.2756) described in Physical Review Letters Vol.56, No.25, p.2775-2758 (1996), by J. Tersoff.
The value of E.sub.FS is known to be 0.5 eV for GaAs and 1.05 eV for AlAs, for example.
For other substances not described in the above-mentioned reference, the hybridized orbital energy value (.sub..epsilon. h-.sub..epsilon. V) in Table 1 (p.1070) described in Journal of Vacuum Science and Technology B4(4), p.1068-1073 (1986), by W. A. Harrison and J. Tersoff, can be used as the value of E.sub.FS -E.sub.V.
With regard to the mixed crystal semiconductors, on the other hand, the value of E.sub.FS can be calculated using Vegard's Rule from the values of semiconductors of various configurations and the composition ratio thereof.
The empirical formulae (1) and (2) described in the above-mentioned references have so far been widely considered to provide a basic concentration limit for well-known semiconductors.
In order to realize a still higher-concentration doping, on the other hand, several attempts have been made. Taking into consideration the fact that the self-compensation in GaAs triggers the generation of Ga vacancy, for example, the Si doping to the extent of 2.times.10.sup.19 /cm.sup.3 has been realized by low-temperature crystal growth without giving the activation energy required for generation of Ga vacancy. A high-concentration doping is realized also by planar doping different from normal doping. (Review "Semiconductor Science and Technology, 9", p.1749-1762 (1994), by R. C. Newman)
It has been shown, on the other hand, that the impurities are readily diffused by heat treatment in these high-concentration doped crystals and their qualities are affected. In all of the above-mentioned methods, the semiconductor is fabricated under a highly non-equilibrium condition, and therefore the semiconductor is not thermally stable. Thus there has been a demand for a thermally stable semiconductor doped with a concentration higher than the conventional limit.
In the case of GaAs or AlGaAs, for example, a high-concentration doped semiconductor that can be fabricated preferably at not lower than 550.degree. C., or more preferably, at not lower than 600.degree. C. and a method of fabrication thereof have been sought after.